A fluid flow conveyed through a flow channel is deflected parallel to the side wall by means of a lateral pressure gradient. Since flow layers that are close to the wall—owing to their lower velocity—are deflected more markedly than flow layers that are far away from the wall, a secondary flow is formed that is superimposed over a main flow. Among other things, this leads to pressure losses. The secondary flows occur regularly in blade cascades of continuous-flow machines such as gas and steam turbines. The blade cascades consist of a plurality of vanes or blades which are arranged next to each other in the circumferential direction in a rotation-symmetrical flow channel and between each of which a blade channel is formed in which the secondary flows arise. The blade channels are each delimited in the radial direction by a radially outer side wall on the housing side or by an outer delimiting wall as well as by a radially inner side wall on the hub side or by an inner delimiting wall. The delimiting walls are formed by a stationary housing section, by a rotor section, by radially inner blade cover plates and/or by radially outer blade cover plates. In the circumferential direction, the blade channels are each delimited by a pressure-side wall and by a suction-side wall of the adjacent blades, In order to reduce the secondary flows or channel vortexes, non-axisymmetrical contours in the form of elevations and/or depressions are often created in the side walls.
Numerous non-axisymmetrical side wall contours are known from the state of the art. Thus, for example, U.S. Pat. Appln. 2008/0026772 A1 proposes arranging the highest section of a pressure-side elevation upstream from the lowest section of a suction-side depression. U.S. Pat. Appln. 2006/0233641 A1 describes a side wall contour in which a pressure-side elevation and a suction-side depression extend over the entire length of the axial blade. U.S. Pat. Appln. 2010/0080708 A1 discloses a side wall contour with a pressure-side elevation whose highest section is arranged in the area of the leading edge, and with a suction-side depression whose lowest section is arranged in the area of the greatest crosswise extension of the blade. German patent application 10 2008 021 053 A1 shows a side wall contour in which a pressure-side elevation extends over an area of 0% to 60% of a blade profile chord, and a suction-side depression extends over an area of 20% to 75% of the profile chord. Here, the highest section of the elevation and the lowest section of the depression are arranged opposite from each other. U.S. Pat. Appln. 2011/0014056 A1 describes a side wall contour whose pressure-side elevation is located over an area of 40% to 80% in the axial extension of the blade and whose suction-side depression is located over an area of 0% to 40% in the axial extension of the blade. Here, the highest section of the elevation is arranged opposite from the lowest section of the depression. Moreover, a second pressure-side elevation and a second suction-side depression can be provided whose outermost sections are arranged opposite from each other. These prior-art side wall contours lead to a reduction of the secondary flows, but they still leave quite a bit of room for improvement.